1. Field of the Invention
This invention relates to a fusible element support assembly for a current limiting fuse wherein a terminal bracket rigidly interconnects a plurality of mica plates in spaced disposition from each other and provides an electrical terminal between the fusible elements and end cap assemblies mounted on each end of a fuse housing. The terminal bracket has flexible structure for permitting the length of the housing to vary due to thermal expansion and contraction while maintaining an electrical and mechanical bond between the end cap assemblies and the fusible elements.
2. Description of the Prior Art
Current limiting fuses are well known in the art and typically comprise an elongated, tubular housing of electrically insulative material, a pair of electrically conductive end caps or terminals that are secured in covering relation to each end of the housing, as well as one or more fusible elements that are disposed within the housing and are electrically coupled to both of the end caps. In order to reduce the overall length of high voltage fuses, it is common practice to form the fusible element or elements to a helical configuration that extends longitudinally within the housing. As such, it is normally necessary to provide a support assembly within the housing that carries the fusible elements in such a fashion that adjacent turns of the element are maintained in spaced relationship to each other and are also retained in a predetermined distance from the interior walls of the housing.
One of the problems associated with support assemblies for high voltage current limiting fuses stems from the fact that electrically insulative support plates used for carrying the fusible elements are typically comprised of mica or mica particles which cause each individual support plate to be somewhat weak and yet quite flexible, and thus be readily susceptible to damage from mechanical stresses during assembly. Micaceous plates also have a relatively low hardness and consequently attempts to mechanically secure the plates to other structure have experienced limited success.
One type of support assembly for a current limiting fuse is shown in U.S. Pat. No. 4,057,775, dated Nov. 8, 1977 and includes two support plates of thin, sheet-like material each having a slot formed along the central axis and extending from one end of each plate to the middle thereof. The plates of the support structure shown in U.S. Pat. No. 4,057,775 are joined in perpendicular orientation so that the structure of both plates which defines respective slots engages each other. Two terminal brackets each have an aperture of an hourglass shape which recieves a narrowed end of the plate, and each bracket provides an electrical connection between a fusible element carried by the support assembly and end cap terminals mounted on the outside of a fuse housing. However, the strength of assembled the support structure shown in U.S. Pat. No. 4,057,775 is somewhat limited and careful handling is necessary during assembly.
Another problem associated with current limiting fuses is the thermal expansion and contraction that can occur when the fuse housing is subjected to temperature excursions, since the overall thermal co-efficient of expansion of the support assembly is often significantly different from the thermal co-efficient of expansion of the fuse housing. When the fuse is exposed to temperature fluctuations, any variation of the distance between the cap assemblies which are securely affixed to opposite ends of the fuse housing can subject the electrical path between the cap assemblies and the fusible elements to severe mechanical stress, often to the point of failure. U.S. Pat. No. 4,146,862, dated Mar. 27, 1979, illustrates an oil-immersible fuse wherein terminal brackets of an internal support assembly are electrically connected to an end cap by two tabs; however, it is believed that the provision of the two tabs shown in this reference does not satisfactorily accomodate stresses imposed on the internal components of the fuse when the latter is exposed to variations in temperature.
It would be a desirable advance in the art if a support assembly for fusible elements of a current limiting fuse was provided which presented a rigid structure when the component parts were assembled even though certain of the parts comprised relatively flexible and weak plates of micaceous material. Such a support assembly would desirably retain the electrical connection between the fusible element and end cap terminals mounted on a fuse housing even though the housing undergoes expansion and contracting during thermal cycling.